Project Summary Lymphatic malformations (LMs) are localized areas of abnormal development of the lymphatic system that commonly occur in the head and neck of children. LMs are considered a rare or orphan disease which causes complications including pain, ulceration, secondary infection, infiltration of other organs, and death. Current therapies involve surgical excision or methods of chemical or physical destruction of portions of lesions. No therapies are uniformly effective and all have the risk of significant adverse events. We recently witnessed almost complete resolution of a LM lesion in a child who was treated with sildenafil oral therapy for pulmonary arterial hypertension. We have subsequently evaluated additional subjects who improved with sildenafil. The goal of this clinical research trial is to document the benefit or absence of benefit of sildenafil therapy for LMs and identify which type of patient will benefit from sildenafil. This study is a double-blind placebo-controlled trial which involves precise documentation of volume changes associated with therapy or placebo by using MRI segmentation techniques. We will also observe and document the clinical response to sildenafil or placebo using clinical evaluation scores and surveys. The results of the study should identify characteristics of LM lesions which may suggest a beneficial response to sildenafil therapy. Sildenafil has very low risk of side effects in the dosage used in this trial. Documentation of an effective response of LMs to sildenafil will accelerate the interest in, and the ability to understand, the mechanisms of LM formation and treatment.